Ok, so I agreed to pick up my sister's friend's little girl from the babysitter today. Well, they gave me a HB booster for her. She is either 3.5 or 4.5. I do not have any extra harnessed seats. I could adjust the Nautilus to fit her and let my DS ride in his booster or just let her ride in the booster for the 15 minutes we will be in car. I will do this twice this week and would have to adjust the Nautilus each time since DS is on the top slot and I would assume she is on te 3rd slot but I haven't seen her since April.